The brutal tactics used in Syria's war

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The ringleader, Abu Ahmed al-Iraqi, may be leader of Islamic State in Iraq and Syria

ISIS is a jihadist group opposed to the regime of Syrian President Bashar al-Assad

Al Qaeda leader Ayman Al-Zawahiri calls for the group to leave Syria

The video clip lasts a mere 35 seconds. It's night. Six men, illuminated by a flashlight, are sitting with their hands tied behind their backs and with various types of blindfolds.

More men huddle behind them in the dark. In front of them is a pit, apparently freshly dug, and on the other side of the pit sits another man similarly bound and blindfolded.

Two seconds into the clip, someone says, "Jahiz?" "Ready?"

At three seconds a man extends his right arm into the light, holding a pistol.

At six seconds the first shot is fired into the back of the first man's head. The man with the pistol moves decisively, without hesitation, shooting one after the other in the back of the head. It would appear he has done this before many times.

Photos:Photos: ISIS reign of fear in Syria

Photos:Photos: ISIS reign of fear in Syria

ISIS reign of fear in Addana, Syria – The checkpoint at the entrance to the Syrian town of Addana is where members of the Islamic State in Iraq and Syria, or ISIS, used to dump the bodies of those they had executed to remind others of the consequences of violating their harsh rules. Now that ISIS has left the town, a flag representing the Islamic Front flies over the checkpoint.

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Photos:Photos: ISIS reign of fear in Syria

ISIS reign of fear in Addana, Syria – ISIS moved into the conservative town about a year ago and was initially welcomed by rebels fighting the Syrian government. Once entrenched, ISIS began enforcing strict rules.

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Photos:Photos: ISIS reign of fear in Syria

ISIS reign of fear in Addana, Syria – People shop and sell goods in the town center midday in Addana. Under ISIS rule, shops were forced to close for the noon prayers, and activities such as smoking were banned. Those who violated the rules were executed or just disappeared.

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Photos:Photos: ISIS reign of fear in Syria

ISIS reign of fear in Addana, Syria – A young boy stands guard near what used to be an ISIS jail. This area that overlooks the town was used for training and executions.

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Photos:Photos: ISIS reign of fear in Syria

ISIS reign of fear in Addana, Syria – Black paint, used by ISIS to mark its territory, can still be seen on the entrance to the jail area. ISIS painted many of the buildings in the town black to announce its presence, and the locals have recently repainted many of the buildings white.

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Photos:Photos: ISIS reign of fear in Syria

ISIS reign of fear in Addana, Syria – This warehouse was used as a jail to hold and execute prisoners. A mass grave was found behind it, and bullet holes mark its walls.

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Photos:Photos: ISIS reign of fear in Syria

ISIS reign of fear in Addana, Syria – Residents dig up the mass grave behind the jail in an effort to identify those killed and to bury them properly.

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Photos:Photos: ISIS reign of fear in Syria

ISIS reign of fear in Addana, Syria – A shoe, a severed foot and several socks are uncovered at the grave site. This is significant to the residents because a missing man was last seen in the market buying socks for his family.

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Photos:Photos: ISIS reign of fear in Syria

ISIS reign of fear in Addana, Syria – The bodies of three brothers were also discovered in the grave.

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Photos:Photos: ISIS reign of fear in Syria

ISIS reign of fear in Addana, Syria – This man came to identify the bodies of his brothers. He knew two of them were dead, but he had hoped that one was still alive and being held elsewhere by ISIS.

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Eleven seconds into the video clip, you can hear what sounds like open faucets. It's the sound of blood gushing from the head wounds. And the gushing sound gets louder and louder.

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Twenty-six seconds after the first shot is fired, a total of 14 men -- some of whom appear to be teenagers -- are dead or dying, some slumping into the pit.

This video clip was one of a variety recently obtained by CNN from Syrian activists. They document atrocities committed not by the regime of President Bashar al-Assad, but rather by members of the Islamic State in Iraq and Syria, known as ISIS, which emerged last year as a major power in opposition-controlled parts of northern Syria.

In the areas controlled by ISIS, public floggings and executions have become commonplace. Most recently ISIS has battled other opposition groups in fighting that has left well over 2,000 people dead.

The material provided by the activists was found in a house in Aleppo occupied by one "Abu Ahmed Al-Iraqi," or "the Iraqi." As is so often the case with members of these radical groups, this is not his real name. He abandoned the video clips, as well as the camera they were shot with, when ISIS retreated from the area during fighting against other opposition groups.

Activists describe Abu Ahmed as an "amir," a commander, responsible for carrying out justice, ISIS style. He is, in effect, judge, jury and executioner. He appears to be the man who carried out the executions described above.

The other clips include eight interrogations of men, most of whom are members of opposition factions not affiliated with ISIS, all operating in or around the town of Al-Bab, northeast of Aleppo. It's not clear whether Abu Ahmed Al-Iraqi is doing the interrogations, but the strong Iraqi accent of the interrogators leaves no doubt about where they come from.

No torture occurs on camera, but it's clear the subjects are under duress. They are all blindfolded, and several sweat profusely. The line of questioning is persistent, always focusing on the activities and opinions of opposition members. Not once in the course of the lengthy interrogations is a single question posed about the Assad regime. In fact, it's not even mentioned.

There are veiled threats; one of the men, who identifies himself as a doctor named Basil, is asked if he ever wants to see his two children again. Another young man, Mohamed, is told to speak the truth to "save himself." He responds by saying, "I will speak the truth, even if I lose my head."

ISIS is clearly worried that many Syrians view them as unwanted interlopers. Thousands of young men have flocked from around the world, including Europe and North America, to join the fight against the Assad regime, and many of the more radical among them have joined ISIS and other jihadi groups.

At one point the interrogator demands of the aforementioned Mohamed: "Didn't people say ISIS are strangers? That they wear "chabounat?" Chabounat is a derisive term referring to the short jalabiyas, or robes, worn by hard-line Islamists to copy the attire of the Prophet Mohammed. Chabounat in this case might be the equivalent of petticoats. Mohamed wisely claims complete ignorance.

From the video clips, it appears that Abu Ahmed has made a life for himself in Syria.

In a series of clips he is instructing a young woman in a black coat how to use an AK-47 assault rifle and a pistol. Her face is exposed -- she is not wearing the "niqab," which completely covers the face and has become de rigueur dress for women in areas controlled by ISIS. She speaks with a Syrian accent.

He's also getting around in a late-model black BMW SUV. And in another video clip, he is doing target practice in the garden of a high-walled villa.

A fancy car, nice house, interrogations and mass executions: meet the new boss in this part of Syria. Same as the old boss.